Process for preparing cemented ferrochrome

ABSTRACT

There is described a cemented ferrochrome material including from 15 to 99 percent of ferrochrome and from 1 to 85 percent of a metallic binder. The material is made by mixing or milling together particles of ferrochrome and the binder metal until the particle size of the mixture is substantially less than 325 mesh, compacting and heating to a temperature within the range of 1850* - 2400*F. The product has excellent corrosion resistance, high hardness, good strength and is highly impermeable to liquids. It may be used in any applications in which such properties are desirable, such as a ball pen balls, wear pads and mounts adapted to be adhered to materials having compatible coefficients of thermal expansion.

llnited States Patent n91 Hill [ 1 Jan.2,1973

[s4] rnocnss ron PREPARING cEMEN'rnp rnnnocnaomn 1970, abandoned.

[52] 11.5. CI ..75/200, 29/182 [51] lint. Cl. ..B22i 3/12 [58] Field of Search ...75/200, 208; 29/18, 23, 182.2, 29/182, 182.1; 148/126 [56] References Cited UNITED STATES PATENTS 2,370,396 2/1945 Cordiano ..75/2l4 OTHER PUBLICATIONS Eisenkolb, Stahl und Eisen, Vol. 78, No. 3, Feb. 6, 1958, pp. 141-148, T5 300.87.

Eisenkolb, Powder Metallurgy, 1961, pp. 75-95, TN 695.?54.

ASM Metals Handbook, 1948 Ed., pp. 338-339, TA 472, A3, 1948.

Primary Examiner-Carl D. Quarforth Assistant Examiner-B. Hunt Attorney-Howard M. I-Ierriot, John E. Fabke and Bacon & Thomas 7 ABSTRACT There is described a cemented ferrochrome material including from 15 to 99 percent of ferrochrome and from 1 to 85 percent of a metallic binder. The material is made by mixing or milling together particles of ferrochrome and the binder metal until the particle size of the mixture is substantially less than 325 mesh, compacting and heating to a temperature within the range of l850 2400F. The product has excellent corrosion resistance, high hardness, good strength and is highly impermeable to liquids. It may be used in any applications in which such properties are desirable, such as a ball pen balls, wear pads and mounts adapted to be adhered to materials having compatible coefiicients of thermal expansion.

4 Claims, No Drawings PROCESS FOR PREPARING CEMENTED FERROCHROME CROSS-REFERENCE TO RELATED APPLICATIONS This is a continuation-in-part of co-pending application Ser. No. 14,527, filed Feb. 26, 1970, now abandoned.

The present invention relates to a cemented ferrochrome material having particularly advantageous properties and to a powder metallurgy process for preparing the same.

It has been proposed to make stainless steels by powder metallurgy wherein relatively minor amounts of a ferrochrome powder were included as an alloying ingredient. Examples of such prior art materials and processes are to be found in the US. Pats. of Kalischer No. 2,333,573, Cordiano No. 2,370,396 and Carlson et al No. 2,827,407.

It is the object of the present invention to provide a cemented ferrochrome material in the form of a composite mass in which particles of ferrochrome retain their individual identity and are bound together by a metallic binder having a melting point lower than that of the ferrochrome.

It is another object of the invention to provide a cemented ferrochrome material characterized by excellent corrosion resistance, high hardness and good strength.

Another object of the invention is to provide a cemented ferrochrome material of low porosity with a resulting high impermeability.

A further object of the invention is to provide a cemented ferrochrome material having a low coefficient of thermal expansion.

Another and further object of the invention is to provide a cemented ferrochrome material which is of low cost as compared to other powder metal products with similar corrosion resistant properties.

A still further object of the invention is to provide a novel process for preparing a cemented ferrochrome material having the advantageous properties set forth above.

Other objects of the invention will become apparent from the following detailed description in conjunction with the accompanying drawings in which:

FIG. 1 is a photomicrograph of a product according to the invention taken at a magnification of 250 times; and

FIG. 2 is a photomicrograph of this same product taken at a magnification of 1250 times.

The product of the invention comprises a composite mass of finely divided ferrochrome particles cemented together by a binder metal or alloy having a melting point lower than that of the ferrochrome. The product may contain from to 99 percent by weight of ferrochrome and from 1 to 85 percent by weight of binder, but it is preferred that the ferrochrome be present in an amount between 25 and 95 percent by weight and the metallic binder in an amount between about 5 and 75 percent by weight. As the percentage of ferrochrome is decreased, the hardness decreases, but the strength and toughness are increased. Accordingly, a balance will be struck in accordance with the particular use to which a given product is to be put.

As will be seen hereinafter, temperatures are employed in the making of the product which are above the melting point of the binder and at the sintering temperatures of the ferrochrome. Accordingly, particles of ferrochrome will be sintered at their points of contact. There will be very little contact between particles of ferrochrome in products which are high in binder and proportionately more in products which are high in ferrochrome.

The metallic binder substantially completely fills all of the spaces between particles of ferrochrome. The product can be made to have an extremely low porosity and thus is to all practical purposes impermeable to liquids of every description. It is to be stressed that the ferrochrome to a large extent maintains its separate identity so that the product is a composite one, although some alloying may take place and the smaller ferrochrome particles may dissolve, at least partially, in the liquid binder.

The size of the ferrochrome particles is quite important. They should have a distribution ranging from less than 1 micron to 50 microns. Particularly good products are those in which the ferrochrome particles prior to being subjected to sintering temperature during fabrication were of a size such that percent by weight would pass through a 325 mesh screen. A very satisfactory product is one in which 99 percent by weight of the ferrochrome particles are of a size less than 20 microns.

The ferrochrome of the material according to the invention should contain from about 60 to 75 percent by weight of chromium, from about 25 to 40 percent by weight of iron and from about 0.1 to 7 percent carbon. There can be used any of the ferrochromes commercially available as an alloying ingredient, these ferrochromes being sold in particulate form, although not in the finely divided form desired in the present products. Both high carbon and low carbon ferrochromes, as well as one containing silicon, may be used in making the present products, but a high carbon ferrochrome is preferred.

A particularly preferred high carbon ferrochrome is one containing about 70 percent by weight of chromium, 25 percent by weight of iron and 5 percent by weight of carbon. The following is a typical screen analysis in weight percent of a commercial ferrochrome of this description:

Mesh Size Ferrochrome and its alloys with palladium and with palladium and manganese also form suitable binders. In certain cases, some of the binder alloy constituents necessary for forming the liquid phase may come from alloying with the ferrochrome.

A particularly suitable binder for preparing the materials of the invention is a mixture of iron and cast iron. The iron is the most economical of the binder materials and the cast iron, because of its low melting point, promotes the liquid phase sintering which occurs in the process of manufacture which will be described in detail hereinafter. Substantially pure iron powders, such as ANCOR MH-l sponge iron powder, manufactured by the Hoeganaes Sponge lron Corporation of Riverton, N. J., and cast iron powders are commercially available. Typical screen analyses of available iron and cast iron powder in weight percent are set forth as follows:

Although either the iron or cast iron powder can be used separately, superior properties are obtained from mixtures of the two; the preferable range of the iron powder being 50 to 90 percent by weight. It is to be understood, of course, that in the final cemented ferrochrome product, the iron and cast iron will have combined to form a steel. Alternatively, a steel powder can be used directly in place of a mixture of iron and cast iron powders.

The products of the invention are made by a novel powder metallurgy technique which ensures the obtaining of a material having the advantageous properties set forth above. The as-received ferrochrome and metallic binder powders are milled together to provide an intimate mixture of finely divided ferrochrome and binder particles. This can suitably be done in a ball mill and milling should preferably be continued until the particle size of the resulting mixture is such that 90 percent will pass through a 325 mesh screen. Milling may be continued until 99 percent of the particles are less than 20 microns in size. Milling will ordinarily be carried out for a period of from 24 to 100 hours depending on equipment used.

The resulting intimate mixture of finely divided powders is then compacted in a mechanical or hydraulic press approximately to the shape of the desired final article under a pressure of to 40 tons/sq. in. This may be accomplished with the aid of a conventional lubricant or organic resin binder, a suitable lubricant comprising about 2 weight percent of paraffin wax, the powders being coated with the lubricant or binder prior to compacting. If a lubricant or binder is used, it is preferably removed before subjecting the compact to the heating step next to be described.

The compact is heated to a temperature within the range of l850 2400 F in an inert or reducing environment achieved by the establishment of vacuum conditions or by provision of a blanket of an inert gas, such as helium, or by use of a reducing gas, such as dissociated ammonia. The binder particles are converted to a totally liquid phase which substantially completely fills all of the voids or spaces between the particles of ferrochrome. As stated above, there may be sintering of ferrochrome particles, but this is not necessary.

There results a considerable shrinkage in going from the green compact to the fully sintered part. For fully cemented ferrochrome, there is ordinarily a 15 to 18 percent linear shrinkage. There results a sintered density of greater than percent of the theoretical and the material has little or no interconnected porosity.

The sintering stage of the process can ordinarily be completed by heating at the stated temperature for from 15 to minutes.

On cooling, there is obtained a hard, dense material which is suitable for most applications. The articles produced can be subjected to conventional finishing operations to obtain the exact size and finish desired. However, if a product of particularly great hardness is required, hardness can be increased by reheating to a temperature of the order of 1500 F in an atmosphere of dissociated ammonia and quenching in oil.

The following examples are set forth as illustrating the invention, but not as limiting the same:

EXAMPLE 1 The following powders were ball milled in a small steel mill with steel balls for 1 12 hours:

40.0 g. ferrochrome 7.5 g. iron 2.5 g. cast iron The milled intimate mixture of powders with a composition of 80 percent by weight of ferrochrome and 20 percent by weight of binder was mixed with an organic resin binder and pelleted into blanks for ball pen balls. The green balls were packed in 60 +100 mesh ZrO and heated slowly to 800 F under dissociated ammonia to remove the binder. The crucible was then transferred to a vacuum furnace and the balls sintered at 2150 F for 30 minutes at 0.2 0.5 torr.

After being ground and finished to size, half of the balls were given a hardening treatment by heating to 1500 F in dissociated ammonia and quenching in oil. Microhardness of the as-sintered balls was DPH 729 and that of the hardened balls was DPH 924. Both types were utilized in ball point pens tested for writing performance and found to be satisfactory. The cemented ferrochrome balls were tested for corrosion resistance as compared to commercial 440C stainless steel ball pen balls by soaking in Super Quink ink at F for 48 weeks. The ferrochrome balls were not affected by the test whereas the 440C stainless steel balls were severely attacked and partially disintegrated.

EXAMPLE 1] The following powders were ball milled in a steel mill with steel balls for 1 12 hours:

80 g. ferrochrome 15 g. iron 5 g. cast iron The resulting mixture of milled powders was mixed with 1 percent by weight of Carbowax 4000 (Union Carbide Corporation) as a lubricant and pressed into discs approximately /s inch thick in a 0.770 inch diameter cylindrical steel die at a pressure of tons/in The green parts were packed in 60 +100 mesh ZrO and sintered in a vacuum furnace at 2200" F for 30 minutes at a pressure of 0.2 0.5 torr. The sintered parts had shrunk to a diameter of 0.645 inches and had a superficial hardness of l5N87. In the final articles, the iron and cast iron had alloyed to provide a steel binder.

The parts showed very little wear and no corrosion when used in the pen nib flexing apparatus as replacement for chromium plated tool steel wear pads'which had chipped and corroded.

FIG. 1 of the drawings is a photomicrograph of the product of this example at a magnification of,250X and FIG. 2 is a photomicrograph of the same product at a magnification of 1250X. As would be expected in a product containing 80 percent by weight of ferrochrome, the continuous skeletal phase is ferrochromium and the binder phase between the particles is steel.

EXAMPLE Ill The procedure of Example 11 was used to prepare sintered discs composed of 85 percent by weight of ferrochrome and 15 percent by weight of steel binder. The following amounts of powder were used:

42.500 g. ferrochrome 5.625 g. iron 1.875 g. white cast iron The sintered discs had shrunk to a diameter of 0.65 inches and had a superficial hardness of 15N85.

EXAMPLE IV The following powders were milled in a steel mill with steel balls for 63 hours:

600 g. ferrochrome 300 g. iron 100 g. white cast iron The mixture of milled powders was mixed with 2 percent by weight paraffin wax and compacted at 10 tons/in in a 2 inch diameter cylindrical steel die. The green parts were packed in 60 mesh alumina and sintered by heating slowly to 2350 F under dissociated ammonia in a continuous pusher type furnace and holding this temperature for one hour. The sintered parts were finished to size by milling and grinding. Testing of the parts showed the material to be impervious to liquid adsorption and to have satisfactory corrosion resistance although the strength was less than that of a similar part made from 316L stainless steel powder.

EXAMPLE v B 800 g. ferrochrome A 800 g. ferrochrome 200 g. white cast iron 200 g. iron 800 g. ferrochrome 650 g. ferrochrome 100 g. iron 175 g. iron 100 g. white cast iron 175 g. white cast iron 500 g. ferrochrome 250 g. iron 250 g. white cast iron The milled powder from each batch was mixed with 2 percent by weight of paraffin wax and standard tensile test specimens (Metal Powder Association Standard 8-50) compacted at 15 tons/in The green specimens were pack e d in 30 mesh alumina an d the wax was removed by heating slowly to 800 F in dissociated ammonia. Final sintering was carried out in a vacuum furnace by heating at 2250 F for 30 minutes at 0.2 0.5 torr. pressure. Properties of the various cemented ferrochromes are given in the following table:

A detailed investigation was conducted on the properties of a series of ferrochrome-steel cemented compositions to determine the useful limits of this system. The percentage by weight of ferrochrome powder was varied from 15 to 100 percent with the binder material, except for the 100 percent ferrochrome composition, being equal weight percentage of iron powder and white cast iron powder. The powders were milled together until the composite powder was essentially less than 37 microns (400 mesh) and at least 95 percent less than 20 microns.

The milled powder was waxed, compacted into transverse rupture test specimens (Metal Powder Association Standard 13-62), and the specimens sintered in either dissociated ammonia or vacuum. Properties of the various compositions were as follows:

Sintertransverse Impact boiling ing Rupture Strength Nitric Atmoshardness Strength ft-lbs/ Acid composition phere rockwell 1000 psi in in/month 15% FeCr' dis. NH R, 67 96.7 19.22 0.657 85% Steel Vac. R 42 137.5 9.45 0.540 20% FeCr dis. NH R 61 83.0 13.67 0.744 80% Steel Vac. R 38 175.1 9.76 0.229 25% FeCr' dis. NH R, 100 181.3 37.50 0.153 Steel Vac. R 33 147.3 19.84 0.146 35% FeCr' dis. NH; R 97 143.8 20.36 0.357 65% Steel Vac. R 40 161.5 18.26 0.182 60% FeCr dis. NH, R 41 112.0 4.62 0.072 40% Steel Vac. R 42 117.1 4.85 0.052 FeCr dis. NH, R 58 50.3 6.52 0.027 20% Steel Vac. R 56 70.7 8.34 0.009 FeCr' dis. NH, R 78 37.9 1.86 0.014 5% Steel Vac. R 81 20.1 0.55 0.010 97% FeCr' dis. NH, R 81 50.9 2.62 0.013 3% Steel Vac. R 80 21.3 0.67 0.067

99% FeCr' dis. NH R 79 48.8 4.00 0.015 1% Steel Vac. R 82 16.1 0.42 0.017 100% FeCr dis. NH; R 79 44.7 1.98 0.048 Vac. R 80 38.8 0.33 0.016

*ASTM Standard Test A262-55T It can be seen that all the compositions have useful properties although for the composites with less than 25 wt. percent ferrochrome the corrosion resistance is considerably reduced. Also the properties of the compositions with 95 wt. percent and greater amounts of ferrochrome do not change to any great extent. In interpreting the strength characteristics of these materials' it should'be notedthat these materials 'fail in a brittle manner and that considerable scatter in test data is to be expected.

The test results show the novel materials of the present invention have properties that are comparable to many commercially available P/M products and that by varying the percentages of the constituents of the compositions it is possible to vary their characteristics over relatively wide limits thereby making them useful for various industrial applications.

EXAMPLE VII The following powders were ball milled in a small steel mill with steel balls for 72 hours:

ferrochrome powder nickel alloy powder (60% by weight of nickel, 20% by weight of manganese and 20% by weight of copper) EXAMPLE VIII The following powders were milled for 1 12 hours:

ferrochrome Monel (70% by weight of nickel and 30% by weight of copper) The mixture of milled powders with a composition of 80 percent by weight of ferrochrome and percent by weight binder alloy was processed into balls as described in Example 1. The balls had a microhardness of DPH 603 and gave satisfactory writing performance in ball pen points.

EXAMPLE 1X The following powders were milled for 1 12 hours:

13.6 g. ferrochrome 3.4 g. cobalt alloy (50% by weight of cobalt, by weight of nickel and 20% by weight of chromium) The mixture of milled powders with a composition of 80 percent by weight of ferrochrome and 20 percent by weight of binder alloy was processed into balls which had a microhardness of DPH 724 and gave satisfactory writing performance in ball pen points.

EXAMPLE X An experiment was conducted using silver as the binder metal for ferrochrome. The powdered metals in ratios to give 10 percent, 20 percent and 30 percent by weight silver composites were mixed together in a mortar, compacted in a 0.25 inch diameter cylindrical steel die at 20 tons/in and sintered in a vacuum furnace at 1850 F for 30 minutes at l torr. pressure of helium. There was little or no shrinkage of the compact but metallographic examination indicates good wetting of the ferrochrome particles by the silver.

EXAMPLE X1 The experiment of Example X was repeated except that palladium powder was added to prepare a composition of 80 percent by weight of ferrochrome, 16 percent by weight of silver and 4 percent by weight of palladium. Sintering at 2200 F resulted in a compact with a good strength but little shrinkage.

EXAMPLE XII The experiment of Example X was repeated except that a prealloyed powder of percent by weight of silver, 20 percent by weight of palladium and 10 percent by weight of manganese was used as the binder metal to prepare a composition of 70 percent by weight of ferrochrome and 30 percent by weight of the silverpalladium-manganese alloy. Sintering at 2350 F for 15 minutes at 200 torr. helium pressure resulted in shrinkage from 0.250 inches to 0.240 inches in diameter. The part had good strength and a hardness value of 30N-75.

There has thus been described the making of cemented ferrochrome materials which are hard, dense, highly impermeable, corrosion resistant and of good strength. The materials have a low coefficient of thermal expansion, generally of the order of 5 microinch/inch/F.

The materials of the invention can be used, in general, for any of the applications for which stainless steel parts formed by powder metallurgy techniques from stainless steel powders have been used. The present materials are particularly suited for the manufacture of ball point pen balls.

Having described my invention, 1 claim:

1. A process for preparing a cemented ferrochrome material, comprising:

a. mixing together 15 to 99 percent by weight of particles of ferrochrome containing about 60 to percent by weight of chromium, from about 25 to 40 percent by weight of iron and from about 0.1 to 7 percent carbon, and l to percent by weight of particles of a binder metal or alloy selected from the group consisting of iron, nickel, cobalt, chromium, manganese, copper, silver, palladium and their alloys, said binder having a melting point lower than that of said ferrochrome until there is obtained an intimate mixture of particles of a size such that 90 percent will pass through a 325 mesh screen;

b. compacting said mixture of particles under pres sure;

0. heating the resulting compact in an inert or reducing environment at a temperature within the range of l850 2400 F such that substantially only said particles of binder melt and substantially fill all of the space between solid ferrochrome particles; and

d. cooling the heated compact in whereby the binder metal binds the ferrochrome particles into a substantially impermeable composite mass.

2. A process as claimed in claim 1 in which the mixture of ferrochrome particles and said particles of metal or alloy are milled until 99 percent of all particles are of a size less than 20 microns.

3. A process as claimed in claim 1 in which said mixture of particles contains from about 25 to 95 percent by weight of said ferrochrome and from about 5 to percent by weight of said metal or alloy.

4. A process as claimed in claim 1 in which said binder metal is a mixture of iron and cast iron. 

2. A process as claimed in claim 1 in which the mixture of ferrochrome particles and said particles of metal or alloy are milled until 99 percent of all particles are of a size less than 20 microns.
 3. A process as claimed in claim 1 in which said mixture of particles contains from about 25 to 95 percent by weight of said ferrochrome and from about 5 to 75 percent by weight of said metal or alloy.
 4. A process as claimed in claim 1 in which said binder metal is a mixture of iron and cast iron. 